Revealed: Why the Soviet Union's Entry into the Pacific War Matters

The Second World War was an unparalleled calamity for the Soviet Union. As many as 27 million Soviet soldiers and civilians died as a result of the conflict that started with the German invasion of Poland in September 1939 and ended with the Japanese surrender in August 1945.

Consumed by this existential struggle along its western border, the Soviet Union was a comparatively minor factor in the Pacific War until the very end. Yet Moscow’s timely intervention in the war against Japan allowed it to expand its influence along the Pacific Rim.

With the breakdown of Allied unity soon heralding the onset of the Cold War, Soviet gains in Asia also left a legacy of division and confrontation, some of which endure into the present.

By the 1930s, Stalin’s Soviet Union and Imperial Japan both viewed themselves as rising powers with ambitions to extend their territorial holdings. In addition to a strategic rivalry dating back to the 19th century, they now nursed an ideological enmity born of the Bolshevik Revolution and the ultraconservative military’s growing hold on Japanese politics. In 1935, Japan signed the Anticomintern Pact with Hitler’s Germany, laying the foundation for the creation of the Axis (Fascist Italy would join the following year).

The two militaries engaged in a series of skirmishes along the frontier between Soviet Siberia and Japanese-occupied Manchuria (Manchukuo) during the late 1930s. The largest, at Khalkin Gol in the summer of 1939, left more than 17,000 dead. Yet worried by growing tensions in Europe and Southeast Asia, both Moscow and Tokyo recognized that their respective ambitions in Manchuria were not worth the mounting costs and soon turned their attention to other theaters.

Just two days after the German Wehrmacht launched Operation Barbarossa in June 1941, Moscow and Tokyo signed a non-aggression pact. Freed from the danger of a two-front war, the Soviet Union was able to focus all its resources on resisting the German onslaught. The Red Army consequently played virtually no role in the Pacific war that was soon raging, at least until the very end.

While recognizing that Moscow had no resources to spare as long as its troops were tied down in Europe, U.S. President Franklin Roosevelt nonetheless sought to enlist Soviet assistance in the war against Japan once Germany had been defeated. Soviet leader Josef Stalin agreed, aiming to expand Soviet borders in Asia. Stalin began building up Soviet forces in the Far East once the tide of the war in Europe had turned following the Battle of Stalingrad.

At the February 1945 Yalta Conference, Stalin agreed that the Soviet Union would enter the war against Japan three months after Germany’s surrender. The Yalta declaration gave Moscow back southern Sakhalin, which Japan had seized during the Russo-Japanese War in 1904-05, as well as the Kurile Island chain to which Russia had renounced its claim in 1875. Mongolia was also to be recognized as an independent state (it was already a Soviet client), and Soviet interests in the naval base at the Chinese port of Port Arthur (Dalian) and the Manchurian railway that it had controlled before 1905 were to be respected.

Moscow subsequently declared war on Tokyo on August 8, 1945, two days after the atomic bombing of Hiroshima and one day before the second bomb fell on Nagasaki (though Western historiography has long emphasized the role of the nuclear attacks in compelling Japan’s surrender, newly available Japanese documents emphasize the importance of the Soviet declaration of war in forcing Tokyo’s hand).

A massive invasion of Manchuria began the day after the Soviet declaration of war. Soviet forces also conducted amphibious landings along Japan’s colonial periphery: Japan’s Northern Territories, on Sakhalin Island, and in the northern part of the Korean Peninsula. The Soviet invasion of Manchuria created a haven for Chinese communist forces, who had been fighting both the Japanese and Chiang Kai-shek’s Nationalists, aiding the communists’ eventual triumph in 1948.

Washington and Moscow had agreed in advance to set up a joint trusteeship in Korea with an eye towards establishing Korea, under Japanese colonial rule since 1910, as an independent state. As in Europe, the U.S. and Soviet Union each received an occupation zone, on either side of the 38th parallel. Unable to reach an agreement on a government for both zones, the U.S. and Soviet trustees presided over the establishment of competing Korean governments for the north (Pyongyang) and south (Seoul). The stage was set for the Korean War, which broke out in January 1950 when North Korean forces poured across the 38th parallel, by then an international border.

The Soviet landings in Sakhalin faced significant Japanese resistance, but gradually succeeded in consolidating control over the entire island. Until 1945, Sakhalin was usually divided between a Russian zone in the north and a Japanese zone in the south. Russia and Japan had struggled over this large, sparsely populated island for more than a century, with the 1855 Treaty of Shimoda specifying that Russians could live in the north of the island and Japanese in the south. Japan relinquished its claims in 1875, but then seized the island during the Russo-Japanese War before returning the northern half to Moscow’s control in 1925. With the Treaty of San Francisco, which formally ended the war in the Pacific, Japan ceded all claims to Sakhalin, leaving the island under Soviet control even though Moscow had declined to sign the treaty.

The Soviet refusal to sign was more problematic with regard to a group of small islands northeast of Hokkaido and southwest of Russia’s Kamchatka Peninsula: Iturup, Kunashir, Shikotan, and Habomai. These islands had also been subject of a Russo-Japanese quarrel dating back to the 19th century. Moscow regarded these islands as the southernmost part of the Kurile chain, which Japan had renounced at San Francisco. The treaty neither specified, however, which islands belonged to the Kurile chain, nor recognized Soviet control over them. Japan, backed up by the U.S. argued that the four islands do not belong to the Kuriles, and that the USSR was illegally occupying them.

The dispute over these islands has prevented an agreement formally ending hostilities between Japan and Russia (as the USSR’s legal successor) up to the present. The issue is highly sensitive to nationalist factions in both Moscow and Tokyo, despite periodic efforts by diplomats on both sides to reach an agreement.

With both Russia and Japan increasingly wary of Chinese power in the Asia-Pacific, four sparsely populated outposts at the edge of the Sea of Okhotsk remain in many ways the biggest impediment to a rapprochement between Moscow and Tokyo that could reshape Asian geopolitics.

Meanwhile, the division of Korea has already sparked one major war, along with and untold suffering inside totalitarian North Korea. With 30,000 American troops still stationed in South Korea across the DMZ from an increasingly paranoid, nuclear armed North Korea, the Korean Peninsula remains one of the world’s most dangerous flashpoints.

Stalin’s intervention in the war against Japan came late in the day, but in many ways it continues shaping the Asian security environment six decades later.